This invention relates to transmitters used in industrial fluid processing plants. In particular, this invention relates to an improved arrangement for field programming switches in process fluid transmitters.
Modern industrial process fluid transmitters often have embedded microprocessors and are field programmable to adjust span, zero, alarm, security or other settings.
In some installations, these setting are programmed by temporarily connecting a handheld serial communication device to the transmitter""s two-wire field circuit. It can be complex and time consuming, however, to find and connect the device and enter programming commands via a keyboard on the device.
In other installations, there are programming switches that are easier to program inside the transmitter circuit board housing. This arrangement, however, has certain disadvantages. In order to use these programming switches in the field, the circuit board housing is opened up in the process plant atmosphere. This exposes sensitive transmitter circuitry to atmospheric humidity, moisture or chemicals.
In some transmitters, the problem is addressed by using magnetically actuated reed switches so that the transmitter circuit board housing does not need to be opened. The arrangement with reed switches, however, is expensive to implement and only a small number of switches can be used because of size limitations.
Programming switch circuitry is energized, along with the rest of the transmitter circuitry, by the two-wire field circuit that provides a 4-20 mA current with a voltage range of 10.5-55 volts. The transmitter must be designed to be fully operational on 4 mA and 10.5 volts, or only about 42 milliwatts. This means that any bias or xe2x80x9cpull-upxe2x80x9d resistors for the switch circuitry must be high impedance to limit power consumption. With such high impedance circuits, even small amounts of chemicals, water, or humidity can cause electrical leakage in the switch circuitry. This leakage can cause a false indication to transmitter circuitry that a switch has been actuated.
There is a desire to have a transmitter that has sensor circuit boards in a permanently sealed housing and also multiple programming switches placed outside the permanently sealed circuit board housing. There is also a desire to avoid false indication from programming switch circuits due to contamination, water or humidity in the environment.
An environmentally sealed service block is mounted on mounting ears inside a field wiring compartment of a process fluid transmitter. The service block includes sealed programming jumper assemblies and sealed programming pushbutton switches. The service block also includes a sealed cable that connects to the pushbutton switches, the jumper assemblies and field wiring terminals. The sealed cable has a plug that plugs into a transmitter electrical connector of a permanently sealed transmitter assembly.
Circuitry connected to the jumper assemblies and pushbutton switches is effectively sealed and not subject to malfunction due to contamination present in the field wiring compartment. The transmitter assembly remains permanently sealed when the programming jumper assemblies or pushbutton switches are actuated.